Need your opinion on which cpu/mobo should i get?
Ryzen 5600 $272.24
Ryzen 5660X $297.97
MSI MAG B550M Mortar $148.1
Asus TUF Gaming B550M-Plus $150.95
MSI B550-A Pro $148.5
All 3 boards cover everything I need in terms of features I/O. I'm more after the durability/performance aspect of each. Is mATX inferior to ATX in anyway besides the extra ports and spacing?
nope.
Often the cheapest barebones models are mATX for cost savings, but both of your listed ones are kind of the tier above the ATX option features wise so that's a non-issue.
You're literally in buy the cheapest/one you think looks the best/roll a die territory with no bad options
1) Stick with 5600 non X, basically the same performance because the cache is more important than 200mhz
2) The "best" is probs either the Mortar or TUF due to their slightly stronger VRMs, but they're only mATX and the B550-A Pro can run a 5950X flat stick anyway so really, choose by features like number of M.2 slots, USB ports, SATA ports etc, or perhaps the 2.5Gbit LAN that some of them may have if you have one of those big $$$ more than gigabit switches - whatever features you care about basically as they're all good boards all for basically the same price.
I have the B550-A Pro with a 5600X and love it (specifically wanted ATX nor mATX).
In an ideal money no object world a bigger badder USB-C port that can do 20Gbps instead of 10Gbps would have been nice for the portable SSD I have that can use that USB 3.2 Gen2x2 or whatever alphabet soup the 20Gbps standard is called, but I rarely use that drive and backup everything over gigabit so meh.